Memory Management in Mac OS
Memory Management in Mac OS
Mindfire Solutions
www.mindfiresolutions.com
March 6, 2002
Abstract:
This paper discusses memory management by macintosh operating system. This paper is
a summarized form of “Inside Macintosh: Memory” and is directed towards developers
who are new to Mac development but had previous development experience on other
operating system. After going through this article you will be familiar with memory
architecture in Mac, ways to allocate and deallocate memory, using temporary memory,
A5 world, heap management, heap zones, heap fragmentation and several other features
provided by memory manager.
MEMORY MANAGEMENT IN MAC OS.................................................................. 1
ORGANIZATION OF MEMORY IN MAC OS ......................................................... 3
The System Heap ..................................................................................................... 4
The System Global Variables................................................................................... 4
ORGANIZATION OF MEMORY IN AN APPLICATION PARTITION 1...................................... 4
The Application Stack 1........................................................................................... 6
The Application Heap 1........................................................................................... 6
The Application Global Variables and A5 World 1 .................................................. 7
TEMPORARY MEMORY 1 .............................................................................................. 8
VIRTUAL MEMORY 1.................................................................................................. 10
ADDRESSING MODES 1............................................................................................... 10
HEAP MANAGEMENT.................................................................................................. 10
Relocatable and Nonrelocatable Blocks 1.............................................................. 10
Properties of Relocatable Blocks 1 ........................................................................ 12
Locking and Unlocking Relocatable Blocks 1 ........................................................ 13
Purging and Reallocating Relocatable Blocks ....................................................... 13
MEMORY RESERVATION 1 .......................................................................................... 14
HEAP PURGING AND COMPACTION 1........................................................................... 14
HEAP FRAGMENTATION 1 ........................................................................................... 15
Deallocating Nonrelocatable Blocks 1................................................................... 15
Locking Relocatable Blocks 1................................................................................ 15
Allocating Nonrelocatable Blocks 1....................................................................... 16
DANGLING POINTERS 1............................................................................................... 16
Callback Routines 1............................................................................................... 17
INVALID HANDLES 1 .................................................................................................. 18
Disposed Handles 1............................................................................................... 18
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Memory Management in Mac OS
Empty Handles 1 ................................................................................................... 18
Fake Handles 1 ..................................................................................................... 18
LOW-MEMORY CONDITIONS....................................................................................... 19
Grow-Zone Functions 1......................................................................................... 19
SETTING UP THE APPLICATION HEAP .......................................................................... 19
Changing the Size of the Stack 1............................................................................ 19
Expanding the Heap 1 ........................................................................................... 20
Allocating Master Pointer Blocks 1 ....................................................................... 20
Defining a Grow-Zone Function 1 ......................................................................... 21
ABOUT THE MEMORY MANAGER ...................................................................... 21
TEMPORARY MEMORY 2 ............................................................................................ 21
INSTALLING A PURGE-WARNING PROCEDURE 2 .......................................................... 22
CREATING HEAP ZONES 2........................................................................................... 22
BLOCK HEADERS 2..................................................................................................... 23
VIRTUAL MEMORY 3.............................................................................................. 23
MEMORY MANAGEMENT UTILITIES ................................................................ 24
USING QUICKDRAW GLOBAL VARIABLES IN STAND-ALONE CODE 4 ........................... 24
THE A5 REGISTER 4 ................................................................................................... 25
ACCESSING THE A5 WORLD IN COMPLETION ROUTINES 4 ........................................... 26
ACCESSING THE A5 WORLD IN INTERRUPT TASKS 4 .................................................... 27
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Memory Management in Mac OS
Organization of Memory in Mac OS
When the Macintosh Operating System starts up, it divides the available RAM into two
broad sections. It reserves for itself a zone or partition of memory known as the system
partition. The system partition always begins at the lowest addressable byte of memory
(memory address 0) and extends upward.
All memory outside the system partition is available for allocation to applications or
other software components. In system software version 7.0 and later (or when
MultiFinder is running in system software versions 5.0 and 6.0), the user can have
multiple applications open at once. When an application is launched, the Operating
System assigns it a section of memory known as its application partition. In general, an
application uses only the memory contained in its own application partition.
Fig: Memory organization in Mac with several applications open.
Source:
Inside Macintosh: Memory
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Memory Management in Mac OS
The System Heap
The main part of the system partition is an area of memory known as the system heap. In
general, the system heap is reserved for exclusive use by the Operating System and other
system software components, which load into it various items such as system resources,
system code segments, and system data structures. All system buffers and queues, for
example, are allocated in the system heap.
Hardware device drivers (stored as code resources of type 'DRVR') are loaded into the
system heap when the driver is opened.
The System Global Variables
The lowest part of memory is occupied by a collection of global variables called system
global variables (or low-memory system global variables). The Operating System uses
these variables to maintain different kinds of information about the operating
environment. For example, the Ticks global variable contains the number of ticks
(sixtieths of a second) that have elapsed since the system was most recently started up.
Other low-memory global variables contain information about the current application.
For example, the ApplZone global variable contains the address of the first byte of the
active application’s partition. The ApplLimit global variable contains the address of
the last byte the active application’s heap can expand to include. The CurrentA5
global variable contains the address of the boundary between the active application’s
global variables and its application parameters. Because these global variables contain
information about the active application, the Operating System changes the values of
these variables whenever a context switch occurs.
Usually, when the value of a low-memory global variable is likely to be useful to
applications, the system software provides a routine that you can use to read or write that
value. For example, you can get the current value of the Ticks global variable by
calling the TickCount function.
Organization of Memory in an Application Partition 1
When your application is launched, the Operating System allocates for it a partition of
memory called its application partition.
Your application partition is divided into three major parts:
1. the application stack
2. the application heap
3. the application global variables and A5 world
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Memory Management in Mac OS
Fig: Organization of an application partition
Source:
Inside Macintosh: Memory
The heap is located at the low-memory end of your application partition and always
expands (when necessary) toward high memory. The A5 world is located at the highmemory end of your application partition and is of fixed size. The stack begins at the
low-memory end of the A5 world and expands downward, toward the top of the heap.
There is usually an unused area of memory between the stack and the heap. This unused
area provides space for the stack to grow without encroaching upon the space assigned to
the application heap. In some cases, however, the stack might grow into space reserved
for the application heap. If this happens, it is very likely that data in the heap will become
corrupted. The ApplLimit global variable marks the upper limit to which your heap
can grow. If you call the MaxApplZone procedure at the beginning of your program,
the heap immediately extends all the way up to this limit. If you were to use all of the
heap’s free space, the Memory Manager would not allow you to allocate additional
blocks above ApplLimit. If you do not call MaxApplZone, the heap grows toward
ApplLimit whenever the Memory Manager finds that there is not enough memory in
the heap to fill a request. However, once the heap grows up to ApplLimit, it can grow
no further. Thus, whether you maximize your application heap or not, you can use only
the space between the bottom of the heap and ApplLimit.
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Memory Management in Mac OS
Unlike the heap, the stack is not bounded by ApplLimit. If your application uses
heavily nested procedures with many local variables or uses extensive recursion, the
stack could grow downward beyond ApplLimit. Because you do not use Memory
Manager routines to allocate memory on the stack, the Memory Manager cannot stop
your stack from growing beyond ApplLimit and possibly encroaching upon space
reserved for the heap. However, a vertical retrace task checks approximately 60 times
each second to see if the stack has moved into the heap. If it has, the task, known as the
“stack sniffer,” generates a system error. This system error alerts you that you have
allowed the stack to grow too far, so that you can make adjustments.
Note: To ensure during debugging that your application generates this system error if the
stack extends beyond ApplLimit, you should call MaxApplZone at the beginning of
your program to expand the heap to ApplLimit.
The Application Stack 1
The stack is an area of memory in your application partition that can grow or shrink at
one end while the other end remains fixed. This means that space on the stack is always
allocated and released in LIFO (last-in, first-out) order. The last item allocated is always
the first to be released. It also means that the allocated area of the stack is always
contiguous. Space is released only at the top of the stack, never in the middle, so there
can never be any unallocated “holes” in the stack.
By convention, the stack grows from high memory toward low memory addresses. The
end of the stack that grows or shrinks is usually referred to as the “top” of the stack, even
though it’s actually at the lower end of memory occupied by the stack.
When your application calls a routine, space is automatically allocated on the stack for a
stack frame. A stack frame contains the routine’s parameters, local variables, and return
address.
Note: Dynamic memory allocation on the stack is usually handled automatically if you
are using a high-level development language such as Pascal. The compiler generates the
code that creates and deletes stack
frames for each function or procedure call.
The Application Heap 1
An application heap is the area of memory in your application partition in which space
is dynamically allocated and released on demand. The heap contains virtually all items
that are not allocated on the stack. For instance, your application heap contains the
application’s code segments and resources that are currently loaded into memory. The
heap also contains other dynamically allocated items such as window
records, dialog records, document data, and so forth.
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Memory Management in Mac OS
The Memory Manager does all the necessary housekeeping to keep track of blocks in the
heap as they are allocated and released. Because these operations can occur in any order,
the heap doesn’t usually grow and shrink in an orderly way, as the stack does. Instead,
after your application has been running for a while, the heap can tend to become
fragmented into a patchwork of allocated and free blocks. This fragmentation is known as
heap fragmentation.
One result of heap fragmentation is that the Memory Manager might not be able to satisfy
your application’s request to allocate a block of a particular size. Even though there is
enough free space available, the space is broken up into blocks smaller than the requested
size. When this happens, the Memory Manager tries to create the needed space by
moving allocated blocks together, thus collecting the free space in a single larger block.
This operation is known as heap compaction.
Heap fragmentation is generally not a problem as long as the blocks of memory you
allocate are free to move during heap compaction. There are, however, two situations in
which a block is not free to move: when it is a nonrelocatable block, and when it is a
locked, relocatable block.
The Application Global Variables and A5 World 1
Fig: Organization of an application’s A5 world
Source:
Inside Macintosh: Memory
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Memory Management in Mac OS
Your application’s global variables are stored in an area of memory near the top of your
application partition known as the application A5 world. The A5 world contains four
kinds of data:
1. application global variables
2. application QuickDraw global variables
3. application parameters
4. the application’s jump table
Each of these items is of fixed size, although the sizes of the global variables and of the
jump table may vary from application to application.
The system global variable CurrentA5 points to the boundary between the current
application’s global variables and its application parameters. For this reason, the
application’s global variables are found as negative offsets from the value of
CurrentA5. This boundary is important because the Operating System uses it to access
the following information from your application: its global variables, its QuickDraw
global variables, the application parameters, and the jump table. This information is
known collectively as the A5 world because the Operating System uses the
microprocessor’s A5 register to point to that boundary.
Your application’s QuickDraw global variables contain information about its drawing
environment. For example, among these variables is a pointer to the current graphics port.
Your application’s jump table contains an entry for each of your application’s routines
that is called by code in another segment. The Segment Manager uses the jump table to
determine the address of any externally referenced routines called by a code segment.
The application parameters are 32 bytes of memory located above the application
global variables; they’re reserved for use by the Operating System. The first long word of
those parameters is a pointer to your application’s QuickDraw global variables.
Temporary Memory 1
In the Macintosh multitasking environment, each application is limited to a particular
memory partition (whose size is determined by information in the 'SIZE' resource of
that application). The size of your application’s partition places certain limits on the size
of your application heap and hence on the sizes of the buffers and other data structures
that your application uses. In general, you specify an application partition size that is
large enough to hold all the buffers, resources, and other data that your application is
likely to need during its execution.
If for some reason you need more memory than is currently available in your application
heap, you can ask the Operating System to let you use any available memory that is not
yet allocated to any other application. This memory, known as temporary memory, is
allocated from the available unused RAM; usually, that memory is not contiguous with
the memory in your application’s zone.
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Memory Management in Mac OS
Fig: Using temporary memory allocated from unused RAM
Source:
Inside Macintosh: Memory
Your application should use temporary memory only for occasional short-term purposes
that could be accomplished in less space, though perhaps less efficiently. For example, if
you want to copy a large file, you might try to allocate a fairly large buffer of temporary
memory. If you receive the temporary memory, you can copy data from the source file
into the destination file using the large buffer. If, however, the request for temporary
memory fails, you can instead use a smaller buffer within your application heap.
One good reason for using temporary memory only occasionally is that you cannot
assume that you will always receive the temporary memory you request.
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Memory Management in Mac OS
Virtual Memory 1
In system software version 7.0 and later, suitably equipped Macintosh computers can take
advantage of a feature of the Operating System known as virtual memory, by which the
machines have a logical address space that extends beyond the limits of the available
physical memory.
It is important to realize that virtual memory operates transparently to most applications.
Unless your application has time-critical needs that might be adversely affected by the
operation of virtual memory or installs routines that execute at interrupt time, you do not
need to know whether virtual memory is operating.
Addressing Modes 1
On suitably equipped Macintosh computers, the Operating System supports 32-bit
addressing, that is, the ability to use 32 bits to determine memory addresses. Earlier
versions of system software use 24-bit addressing, where the upper 8 bits of memory
addresses are ignored or used as flag bits. When 32-bit addressing is in operation, the
maximum program address space is 1 GB.
Heap Management
Relocatable and Nonrelocatable Blocks 1
You can use the Memory Manager to allocate two different types of blocks in your heap:
nonrelocatable blocks and relocatable blocks. A nonrelocatable block is a block of
memory whose location in the heap is fixed. In contrast, a relocatable block is a block of
memory that can be moved within the heap (perhaps during heap compaction).
To reference a nonrelocatable block, you can use a pointer variable, defined by the Ptr
data type.
A pointer is simply the address of an arbitrary byte in memory, and a pointer to a
nonrelocatable block of memory is simply the address of the first byte in the block.
Because a pointer is the address of a block of memory that cannot be moved, all copies of
the pointer correctly reference the block as long as you don’t dispose of it.
The pointer variable itself occupies 4 bytes of space in your application partition. Often
the pointer variable is a global variable and is therefore contained in your application’s
A5 world. But the pointer can also be allocated on the stack or in the heap itself. To
reference relocatable blocks, the Memory Manager uses a scheme known as double
indirection. The Memory Manager keeps track of a relocatable block internally with a
master pointer, which itself is part of a nonrelocatable master pointer block in your
application heap and can never move.
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Memory Management in Mac OS
Note: The Memory Manager allocates one master pointer block (containing 64 master
pointers) for your application at launch time, and you can call the MoreMasters
procedure to request that additional master pointer blocks be allocated.
When the Memory Manager moves a relocatable block, it updates the master pointer so
that it always contains the address of the relocatable block. You reference the block with
a handle, defined by the Handle data type.
A handle is a Memory Manager structure which is basically a pointer to a pointer (the
first pointer is a handle, the second one is called a master pointer). Your code remembers
the handle, and then, to resize it, the Memory Manager can free the master pointer and
allocate new space anywhere in the heap. You allocate a handle using NewHandle and
you release it using DisposeHandle.
One of the most common times that you will use handles is when dealing with resources.
All resources on the Macintosh are allocated as handles. You allocate a resource handle
using GetResource and you release it using ReleaseResource or by closing the resource
file.
Fig: A pointer to a non relocatable block
Source:
Inside Macintosh: Memory
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Memory Management in Mac OS
Fig: A handle to a relocatable block
Source:
Inside Macintosh: Memory
Master pointers for relocatable objects in your heap are always allocated in your
application heap. Because the blocks of masters pointers are nonrelocatable, it is best to
allocate them as low in your heap as possible. You can do this by calling the
MoreMasters procedure when your application starts up.
In some cases, however, you may be forced to allocate a nonrelocatable block of
memory. When you call the Window Manager function NewWindow, for example, the
Window Manager internally calls the NewPtr function to allocate a new nonrelocatable
block in your application partition.
Using relocatable blocks makes the Memory Manager more efficient at managing
available space, but it does carry some overhead. As you have seen, the Memory
Manager must allocate extra memory to hold master pointers for relocatable blocks.
Properties of Relocatable Blocks 1
If relocatable, a block can be either locked or unlocked; if it’s unlocked, a block can be
either purgeable or unpurgeable. These attributes of relocatable blocks can be set and
changed as necessary
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Memory Management in Mac OS
Locking and Unlocking Relocatable Blocks 1
To prevent a block from moving, you can lock it, using the HLock procedure. Once you
have locked a block, it won’t move. Later, you can unlock it, using the Hunlock
procedure, allowing it to move again. In general, you need to lock a relocatable block
only if there is some danger that it might be moved during the time that you read or write
the data in that block. This might happen, for instance, if you dereference a handle to
obtain a pointer to the data and (for increased speed) use the pointer within a loop that
calls routines that might cause memory to be moved. If, within the loop, the block whose
data you are accessing is in fact moved, then the pointer no longer points to that data; this
pointer is said to dangle.
Note : Locking a block is only one way to prevent a dangling pointer.
Using locked relocatable blocks can, however, slow the Memory Manager down as much
as using nonrelocatable blocks. The Memory Manager can’t move locked blocks. In
addition, except when you allocate memory and resize relocatable blocks, it can’t move
relocatable blocks around locked relocatable blocks (just as it can’t move them around
nonrelocatable blocks). Thus, locking a block in the middle of the heap for long periods
of time can increase heap fragmentation.
Locking and unlocking blocks every time you want to prevent a block from moving can
become troublesome. Fortunately, the Memory Manager moves unlocked, relocatable
blocks only at well-defined, predictable times. In general, each routine description in
Inside Macintosh indicates whether the routine could move or purge memory. If you do
not call any of those routines in a section of code, you can rely on all blocks to remain
stationary while that code executes. Note that the Segment Manager might move memory
if you call a routine located in a segment that is not currently resident in memory.
Purging and Reallocating Relocatable Blocks
By making a relocatable block purgeable, you allow the Memory Manager to free the
space it occupies if necessary. If you later want to prohibit the Memory Manager from
freeing the space occupied by a relocatable block, you can make the block unpurgeable.
You can use the HPurge and HNoPurge procedures to change back and forth
between these two states. A block you create by calling NewHandle is initially
unpurgeable. Once you make a relocatable block purgeable, you should subsequently
check handles to that block before using them if you call any of the routines that could
move or purge memory. If a handle’s master pointer is set to NIL, then the Operating
System has purged its block. To use the information formerly in the block, you must
reallocate space for it (perhaps by calling the ReallocateHandle procedure) and
then reconstruct its contents (for example, by rereading the preferences file).
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Memory Management in Mac OS
Fig: Purging and reallocating a relocatable block
Source:
Inside Macintosh: Memory
Memory Reservation 1
The Memory Manager does its best to prevent situations in which nonrelocatable blocks
in the middle of the heap trap relocatable blocks. When it allocates new nonrelocatable
blocks, it attempts to reserve memory for them as low in the heap as possible. The
Memory Manager reserves memory for a nonrelocatable block by moving unlocked
relocatable blocks upward until it has created a space large enough for the new block.
When the Memory Manager can successfully pack all nonrelocatable blocks into the
bottom of the heap, no nonrelocatable block can trap a relocatable block, and it has
successfully prevented heap fragmentation.
During this process, the Memory Manager might even move a relocatable block over a
nonrelocatable block to make room for another nonrelocatable block.
Heap Purging and Compaction 1
When your application attempts to allocate memory (for example, by calling either the
NewPtr or NewHandle function), the Memory Manager might need to compact or
purge the heap to free memory and to fuse many small free blocks into fewer large free
blocks. The Memory Manager first tries to obtain the requested amount of space by
compacting the heap; if compaction fails to free the required amount of space, the
Memory Manager then purges the heap.
When compacting the heap, the Memory Manager moves unlocked, relocatable blocks
down until they reach nonrelocatable blocks or locked, relocatable blocks. You can
compact the heap manually, by calling either the CompactMem function or the
MaxMem function.
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Memory Management in Mac OS
If you want, you can manually purge a few blocks or an entire heap in anticipation of a
memory shortage. To purge an individual block manually, call the EmptyHandle
procedure. To purge your entire heap manually, call the PurgeMem procedure or the
MaxMem function.
Heap Fragmentation 1
Throughout this section, you should keep in mind the following rule: the Memory
Manager can move a relocatable block around a nonrelocatable block (or a locked
relocatable block) at these times only:
When the Memory Manager reserves memory for a nonrelocatable block (or when you
manually reserve memory before allocating a block), it can move unlocked, relocatable
blocks upward over nonrelocatable blocks to make room for the new block as low in the
heap as possible.
When you attempt to resize a relocatable block, the Memory Manager can move that
block around other blocks if necessary.
In contrast, the Memory Manager cannot move relocatable blocks over nonrelocatable
blocks during compaction of the heap.
Deallocating Nonrelocatable Blocks 1
One of the most common causes of heap fragmentation is also one of the most difficult to
avoid. The problem occurs when you dispose of a nonrelocatable block in the middle of
the pile of nonrelocatable blocks at the bottom of the heap. Unless you immediately
allocate another nonrelocatable block of the same size, you create a gap where the
nonrelocatable block used to be. If you later allocate a slightly smaller, nonrelocatable
block, that gap shrinks. However, small gaps are inefficient because of the small
likelihood that future memory allocations will create blocks small enough to occupy the
gaps. It would not matter if the first block you allocated after deleting the nonrelocatable
block were relocatable. The Memory Manager would place the block in the gap if
possible. If you were later to allocate a nonrelocatable block as large as or smaller than
the gap, the new block would take the place of the relocatable block, which would join
other relocatable blocks in the middle of the heap, as desired. However, the new
nonrelocatable block might be smaller than the original nonrelocatable block, leaving a
small gap.
Locking Relocatable Blocks 1
Locked relocatable blocks present a special problem. When relocatable blocks are locked,
they can cause as much heap fragmentation as nonrelocatable blocks. One solution is to
reserve memory for all relocatable blocks that might at some point need to be locked, and
to leave them locked for as long as they are allocated. This solution has drawbacks,
however, because then the blocks would lose any flexibility that being relocatable
otherwise gives them. Deleting a locked relocatable block can create a gap, just as
deleting a nonrelocatable block can.
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Memory Management in Mac OS
Fig: An effectively partitioned heap
Source:
Inside Macintosh: Memory
Allocating Nonrelocatable Blocks 1
As you have seen, there are two reasons for not allocating nonrelocatable blocks during
the middle of your application’s execution. First, if you also dispose of nonrelocatable
blocks in the middle of your application’s execution, then allocation of new
nonrelocatable blocks is likely to create small gaps, as discussed earlier. Second, even if
you never dispose of nonrelocatable blocks until your application terminates, memory
reservation is an imperfect process, and the Memory Manager could occasionally place
new nonrelocatable blocks above relocatable blocks.
Dangling Pointers 1
Accessing a relocatable block by double indirection, through its handle instead of through
its master pointer, requires an extra memory reference. For efficiency, you might
sometimes want to dereference the handle— that is, make a copy of the block’s master
pointer— and then use that pointer to access the block by single indirection. When you do
this, however, you need to be particularly careful. Any operation that allocates space
from the heap might cause the relocatable block to be moved or purged. In that event, the
block’s master pointer is correctly updated, but your copy of the master pointer is not. As
a result, your copy of the master pointer is a dangling pointer. Dangling pointers are
likely to make your application crash or produce garbled output.
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Memory Management in Mac OS
The easiest way to prevent dangling pointers is simply to lock the relocatable block
whose data you want to read or write.
Callback Routines 1
Code segmentation can also lead to a different type of dangling-pointer problem when
you use callback routines. The problem rarely arises, but it is difficult to debug. Some
Toolbox routines require that you pass a pointer to a procedure in a variable of type
ProcPtr. Ordinarily, it does not matter whether the procedure you pass in such a
variable is in the same code segment as the routine that calls it or in a different code
segment. For example, suppose you call TrackControl as follows:
myPart := TrackControl(myControl, myEvent.where,
@MyCallBack);
If MyCallBack were in the same code segment as this line of code, then a compiler
would pass to TrackControl the absolute address of the MyCallBack procedure.
If it were in a different code segment, then the compiler would take the address from the
jump table entry for MyCallBack. Either way, TrackControl should call
MyCallBack correctly.
Occasionally, you might use a variable of type ProcPtr to hold the address of a
callback procedure and then pass that address to a routine. Here is an example:
myProc := @MyCallBack;
...
myPart := TrackControl(myControl, myEvent.where, myProc);
As long as these lines of code are in the same code segment and the segment is not
unloaded between the execution of those lines, the preceding code should work perfectly.
Suppose, however, that myProc is a global variable, and the first line of the code is in a
different segment from the call to TrackControl. Suppose, further, that the
MyCallBack procedure is in the same segment as the first line of the code (which is in
a different segment from the call to TrackControl). Then, the compiler might place
the absolute address of the MyCallBack routine into the variable myProc. The
compiler cannot realize that you plan to use the variable in a different code segment from
the one that holds both the routine you are referencing and the routine you are using to
initialize the myProc variable. Because MyCallBack and the call to
TrackControl are in different code segments, the TrackControl procedure
requires that you pass an address in the jump table, not an absolute address. Thus, in this
hypothetical situation, myProc would reference MyCallBack incorrectly. To avoid
this problem, make sure to place in the same segment any code in which you assign a
value to a variable of type ProcPtr and any code in which you use that variable. If
you must put them in different code segments, then be sure that you place the callback
routine in a code segment different from the one that initializes the variable.
Note: Some development systems allow you to specify compiler options that force jump
table references to be generated for routine addresses. If you specify those options, the
problems described in this section
cannot arise.
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Memory Management in Mac OS
Invalid Handles 1
An invalid handle refers to the wrong area of memory, just as a dangling pointer does.
There are three types of invalid handles: empty handles, disposed handles, and fake
handles.
Disposed Handles 1
A disposed handle is a handle whose associated relocatable block has been disposed of.
When you dispose of a relocatable block (perhaps by calling the procedure
DisposeHandle), the Memory Manager does not change the value of any handle
variables that previously referenced that block. Instead, those variables still hold the
address of what once was the relocatable block’s master pointer. Because the block has
been disposed of, however, the contents of the master pointer are no longer defined. (The
master pointer might belong to a subsequently allocated relocatable block, or it could
become part of a linked list of unused master pointers maintained by the Memory
Manager.)
You can avoid these problems quite easily by assigning the value NIL to the handle
variable after you dispose of its associated block.
Empty Handles 1
An empty handle is a handle whose master pointer has the value NIL. When the
Memory Manager purges a relocatable block, for example, it sets the block’s master
pointer to NIL. The space occupied by the master pointer itself remains allocated, and
handles to the purged block continue to point to the master pointer. This is useful,
because if you later reallocate space for the block by calling ReallocateHandle, the
master pointer will be updated and all existing handles will correctly access the
reallocated block.
Note: Don’t confuse empty handles with 0-length handles, which are handles whose
associated block has a size of 0 bytes. A 0-length handle has a non-NIL master pointer
and a block header.
Fake Handles 1
A fake handle is a handle that was not created by the Memory Manager. Normally, you
create handles by either directly or indirectly calling the Memory Manager function
NewHandle (or one of its variants, such as NewHandleClear). You create a fake
handle— usually inadvertently— by directly assigning a value to a variable of type
Handle, as illustrated in follwing listing.
Listing: Creating a fake handle
FUNCTION MakeFakeHandle: Handle; {DON’T USE THIS FUNCTION!}
CONST
kMemoryLoc = $100; {a random memory location}
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VAR
Low-Memory Conditions
You can take several steps to help maximize the amount of free space in your heap. For
example, you can mark as purgeable any relocatable blocks whose contents could easily
be reconstructed.
Before you call NewHandle or NewPtr, you should check that, if the requested amount
of memory were in fact allocated, the remaining amount of space free in the heap would
not fall below a certain threshold. The free memory defined by that threshold is your
memory cushion.
Grow-Zone Functions 1
The Memory Manager provides a particularly easy way for you to make sure that the
emergency memory reserve is released when necessary. You can define a grow-zone
function that is associated with your application heap. The Memory Manager calls your
heap’s grow-zone function only after other techniques of freeing memory to satisfy a
memory request fail (that is, after compacting and purging the heap and extending the
heap zone to its maximum size). The grow-zone function can then take appropriate steps
to free additional memory. A grow-zone function might dispose of some blocks or make
some unpurgeable blocks purgeable. When the function returns, the Memory Manager
once again purges and compacts the heap and tries to reallocate memory. If there is still
insufficient memory, the Memory Manager calls the grow-zone function again (but only
if the function returned a nonzero value the previous time it was called). This mechanism
allows your grow-zone function to release just a little bit of memory at a time. If the
amount it releases at any time is not enough, the Memory Manager calls it again and
gives it the opportunity to take more drastic measures. As the most drastic step to freeing
memory in your heap,
you can release the emergency reserve.
Setting Up the Application Heap
To help prevent heap fragmentation, you should also perform some setup of your own
early in your application’s execution. Depending on the needs of your application, you
might want to
change the size of your application’s stack
expand the heap to the heap limit
allocate additional master pointer blocks
The following sections describe in detail how and when to perform these operations.
Changing the Size of the Stack 1
Most applications allocate space on their stack in a predictable way and do not need to
monitor stack space during their execution. For these applications, stack usage usually
reaches a maximum in some heavily nested routine. If the stack in your application can
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never grow beyond a certain size, then to avoid collisions between your stack and heap
you simply need to ensure that your stack is large enough to accommodate that size. If
you never encounter system error 28 (generated by the stack sniffer when it detects a
collision between the stack and the heap) during application testing, then you probably do
not need to increase the size of your stack. To increase the size of your stack, you simply
reduce the size of your heap. Because the heap cannot grow above the boundary
contained in the ApplLimit global variable, you can lower the value of ApplLimit
to limit the heap’s growth. By lowering ApplLimit, technically you are not making the
stack bigger; you are just preventing collisions between it and the heap.
By default, the stack can grow to 8 KB on Macintosh computers without Color
QuickDraw and to 32 KB on computers with Color QuickDraw.
Listing below defines a procedure that increases the stack size by a given value. It does
so by determining the current heap limit, subtracting the value of the extraBytes
parameter from that value, and then setting the application limit to the difference.
Listing : Increasing the amount of space allocated for the stack
PROCEDURE IncreaseStackSize (extraBytes: Size);
BEGIN
SetApplLimit(Ptr(ORD4(GetApplLimit) - extraBytes));
END;
You should call this procedure at the beginning of your application, before you call the
MaxApplZone procedure.
Expanding the Heap 1
Near the beginning of your application’s execution, before you allocate any memory, you
should call the MaxApplZone procedure to expand the application heap immediately
to the application heap limit. If you do not do this, the Memory Manager gradually
expands your heap as memory needs require. This gradual expansion can result in
significant heap fragmentation if you have previously moved relocatable blocks to the top
of the heap (by calling MoveHHi) and locked them (by calling HLock). When the heap
grows beyond those locked blocks, they are no longer at the top of the heap. Your heap
then remains fragmented for as long as those blocks remain locked.
Allocating Master Pointer Blocks 1
After calling MaxApplZone, you should call the MoreMasters procedure to allocate
as many new nonrelocatable blocks of master pointers as your application is likely to
need during its execution. Each block of master pointers in your application heap
contains 64 master pointers. The Operating System allocates one block of master pointers
as your application is loaded into memory, and every relocatable block you allocate needs
one master pointer to reference it.
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Defining a Grow-Zone Function 1
The Memory Manager calls your heap’s grow-zone function only after other attempts to
obtain enough memory to satisfy a memory allocation request have failed. A grow-zone
function should be of the following form:
FUNCTION MyGrowZone (cbNeeded: Size): LongInt;
The Memory Manager passes to your function (in the cbNeeded parameter) the
number of bytes it needs. Your function can do whatever it likes to free that much space
in the heap.
PROCEDURE InitializeEmergencyMemory;
BEGIN
gEmergencyMemory := NewHandle(kEmergencyMemorySize);
SetGrowZone(@MyGrowZone);
END;
About the Memory Manager
The Memory Manager provides a large number of routines that you can use to perform
various operations on blocks within your application partition. You can use the Memory
Manager to set up your application partition allocate and release both relocatable and
nonrelocatable blocks in your application heap copy data from nonrelocatable blocks to
relocatable blocks, and vice versa determine how much space is free in your heap
determine the location of the top of your stack determine the size of a memory block and,
if necessary, change that size change the properties of relocatable blocks install or
remove a grow-zone function for your heap obtain the result code of the most recent
Memory Manager routine executed. The Memory Manager also provides routines that
you can use to access areas of memory outside your application partition. You can use the
Memory Manager to n allocate memory outside your partition that is currently unused by
any open application or by the Operating System allocate memory in the system heap
This section describes the areas of memory that lie outside your application partition. It
also describes multiple heap zones.
Temporary Memory 2
In the Macintosh multitasking environment, your application is limited to a particular
memory partition (whose size is determined by information in the 'SIZE' resource of
your application). The size of your application’s partition places certain limits on the size
of your application heap and hence on the sizes of the buffers and other data structures
that your application can use.
If for some reason you need more memory than is currently available in your application
heap, you can ask the Operating System to let you use any available memory that is not
yet allocated to any other application. This memory, called temporary memory, is
allocated from the available unused RAM; in general, that memory is not contiguous with
the memory in your application’s zone Your application should use temporary memory
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only for occasional short-term purposes that could be accomplished in less space, though
perhaps less efficiently.
Installing a Purge-Warning Procedure 2
You can define a purge-warning procedure that the Memory Manager calls whenever it
is about to purge a block from your application heap. You can use this procedure to save
the data in the block, if necessary, or to perform other processing in response to this
notification.
Creating Heap Zones 2
You can create heap zones as subzones of your application heap zone or (in rare
instances) either in space reserved for the application global variables or on the stack.
You can also create heap zones in a block of temporary memory or within the system
heap zone. This section describes how to create new heap zones by calling the
InitZone procedure.
Listing : Creating a subzone of the original application heap zone
FUNCTION CreateSubZone: THz;
CONST
kZoneSize = 10240; {10K zone}
kNumMasterPointers = 16; {num of master ptrs for new zone}
VAR
start: Ptr; {first byte in zone}
limit: Ptr; {first byte beyond zone}
BEGIN
start := NewPtr(kZoneSize); {allocate storage for zone}
IF MemError <> noErr THEN
BEGIN {allocation successful}
limit := Ptr(ORD4(start) + kZoneSize);
{compute byte beyond end of zone}
InitZone(NIL, kNumMasterPointers, limit, start);
{initialize zone header, trailer}
END;
CreateSubZone := THz(start); {cast storage to a zone
pointer}
END;
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Block Headers 2
Fig: A block header in a 32-bit zone
Source:
Inside Macintosh: Memory
Every block in a heap zone, whether allocated or free, has a block header that the
Memory Manager uses to find its way around in the zone. Block headers are completely
transparent to your application. All pointers and handles to allocated blocks reference the
beginning of the block’s logical contents, following the end of the header. Similarly,
whenever you use a variable of type Size, that variable refers to the number of bytes in
the block’s logical contents, not including the block header. That size is known as the
block’s logical size, as opposed to its physical size, the number of bytes it actually
occupies in memory, including the header and any unused bytes at the end of the block.
Virtual Memory 3
A user can select (in the Memory control panel) whether to enable this larger or “virtual”
address space. Most applications are completely unaffected by the operation of the
Virtual Memory Manager and have no need to know whether any virtual memory is
available. You might, however, need to intervene in the otherwise automatic workings of
the Virtual Memory Manager if your application has critical timing requirements,
executes code at interrupt time, or performs debugging operations.
The Virtual Memory Manager extends the logical address space by using part of the
available secondary storage (such as a hard disk) to hold portions of applications and data
that are not currently in use in physical memory. When an application needs to operate on
portions of memory that have been transferred to disk, the Virtual Memory Manager
loads those portions back into physical memory by making them trade places with other,
unused segments of memory. This process of moving portions (or pages) of memory
between physical RAM and the hard disk is called paging.
Users control and configure virtual memory through the Memory control panel. Controls
in this panel allow the user to turn virtual memory on or off, set the size of virtual
memory, and set the volume on which the invisible backing-store file resides. (The
backing-store file is the file in which the Operating System stores the contents of
nonresident pages of memory.)
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Memory Management Utilities
Using QuickDraw Global Variables in Stand-Alone Code 4
If you are writing a stand-alone code segment such as a definition procedure for a
window, menu, or control, you might want routines in that segment to examine the
QuickDraw global variables of the current application. For example, you might want a
control definition function to reference some of the QuickDraw global variables, such as
thePort, screenBits, or the predefined patterns. Stand-alone segments, however,
have no A5 world; if you try to link a stand-alone code segment that references your
application’s global variables, the linker may be unable to resolve those references. To
solve this problem, you can have the definition function find the value of the
application’s A5 register (by calling the SetCurrentA5 function) and then use that
information to copy all of the application’s QuickDraw global variables into a record in
the function’s own private storage. Listing below defines a record type with the same
structure as the QuickDraw global variables. Note that randSeed is stored lowest in
memory and thePort is stored highest in memory.
Listing : Structure of the QuickDraw global variables
TYPE
QDVarRecPtr = ^QDVarRec;
QDVarRec =
RECORD
randSeed: LongInt; {for random-number generator}
screenBits: BitMap; {rectangle enclosing screen}
arrow: Cursor; {standard arrow cursor}
dkGray: Pattern; {75% gray pattern}
ltGray: Pattern; {25% gray pattern}
gray: Pattern; {50% gray pattern}
black: Pattern; {all-black pattern}
white: Pattern; {all-white pattern}
thePort: GrafPtr; {pointer to current GrafPort}
END;
The location of these variables is linker-dependent. However, the A5 register always
points to the last of these global variables, thePort. The Operating System references
all other QuickDraw global variables as negative offsets from thePort. Therefore, you
must dereference the value in A5 (to obtain the address of thePort), and then subtract
the combined size of the other QuickDraw global variables from that address. The
difference is a pointer to the first of the QuickDraw global variables, randSeed. You
can copy the entire record into a local variable simply by dereferencing that pointer, as
illustrated in Listing below.
Listing : Copying the QuickDraw global variables into a record
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PROCEDURE GetQDVars (VAR qdVars: QDVarRec);
TYPE
LongPtr = ^LongInt;
BEGIN
qdVars := QDVarRecPtr(LongPtr(SetCurrentA5)^ (SizeOf(QDVarRec) - SizeOf(thePort)))^;
END;
Thereafter, your stand-alone code segment can read QuickDraw global variables through
the structure returned by GetQDVars. Listing below defines a very simple draw routine
for a control definition function. After reading the calling application’s QuickDraw
global variables, the draw routine paints a rectangle with a pattern.
Listing : A control’s draw routine using the calling application’s QuickDraw patterns
PROCEDURE DoDraw (varCode: Integer; myControl:
ControlHandle;
flag: Integer);
VAR
cRect: Rect;
qdVars: QDVarRec;
origPenState: PenState;
CONST
kDraw = 1; {constant to specify drawing}
BEGIN
GetPenState(origPenState); {get original pen state}
cRect := myControl^^.contrlRect; {get control’s rectangle}
IF flag = kDraw THEN
BEGIN
GetQDVars(qdVars); {patterns are QD globals}
PenPat(qdVars.gray); {install desired pattern}
PaintRect(cRect); {paint the control}
END;
SetPenState(origPenState); {restore original pen state}
END;
The DoDraw drawing routine defined in Listing 4-6 retrieves the calling application’s
QuickDraw global variables and paints the control rectangle with a light gray pattern. It
also saves and restores the pen state, because the PenPat procedure changes that state.
The A5 Register 4
If you write code that accesses your application’s A5 world (usually to read or write the
application global variables) at a time that your application is not the current application,
you must ensure that the A5 register points to the boundary between your application’s
parameters and global variables. Because the Operating System accesses your A5 world
relative to the address stored in the A5 register, you can obtain unpredictable results if
you attempt to read or write data in your A5 world when the contents of A5 are not valid.
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Accessing the A5 World in Completion Routines 4
Some Toolbox and Operating System routines require you to pass the address of an
application-defined callback routine, usually in a variable of type ProcPtr. After a
certain condition has been met, the Toolbox executes the specified routine. The exact
time at which the Toolbox executes the routine varies. The timing of execution is
determined by the Toolbox routine to which you passed the routine’s address and the
action that must be completed before the routine is called. Callback routines are quite
common in the Macintosh system software. A grow-zone function, for instance, is an
application-defined callback routine that is called every time the Memory Manager
cannot find enough space in your heap to honor a memory-allocation request. Similarly,
if your application plays a sound asynchronously, you can have the Sound Manager
execute a completion routine after the sound is played. The completion routine might
release the sound channel used to play the sound or perform other cleanup operations. In
general, you cannot predict what your application will be doing when an asynchronous
completion or callback routine is actually executed. The routine could be called while
your application is executing code of its own or executing another Toolbox or Operating
System routine.
Note: The completion or callback routine might even be called when your application is
in the background. Before executing a completion or callback routine belonging to your
application, the Process Manager checks whether your application is in the foreground. If
not, the Process Manager performs a minor switch to give your application temporary
control of the CPU.
Many Toolbox and Operating System routines do not need to access the calling
application’s global variables, QuickDraw global variables, or jump table. As a result,
they sometimes use the A5 register for their own purposes. They save the current value of
the register upon entry, modify the register as necessary, and then restore the original
value on exit. As you can see, if one of these routines is executing when your callback
routine is executed, your callback routine cannot depend on the value in the A5 register.
This effectively prevents your callback routine from using any part of its A5 world. To
solve this problem, simply use the strategy that the Toolbox employs when it takes over
the A5 register: save the current value in the A5 register at the start of your callback
procedure, install your application’s A5 value, and then restore the original value when
you exit. Listing below illustrates a very simple grow-zone function that uses this
technique. It uses the SetCurrentA5 and SetA5 utilities to manipulate the A5
register.
4
Listing : A sample grow-zone function
FUNCTION MyGrowZone (cbNeeded: Size): LongInt;
VAR
theA5: LongInt; {value of A5 when function is called}
BEGIN
theA5 := SetCurrentA5; {remember current value of A5;
install ours}
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IF (gEmergencyMemory^ <> NIL) & (gEmergencyMemory <>
GZSaveHnd) THEN
BEGIN
EmptyHandle(gEmergencyMemory);
MyGrowZone := kEmergencyMemorySize;
END
ELSE
MyGrowZone := 0; {no more memory to release}
theA5 := SetA5(theA5); {restore previous value of A5}
END;
The function SetCurrentA5 does two things: it returns the current value in the A5
register, and it sets the A5 register to the value of the CurrentA5 low-memory global
variable. This global variable always contains a value that points to the boundary between
the current application’s parameters and its global variables. The MyGrowZone
function defined in Listing 4-1 calls SetCurrentA5 on entry to make sure that it can
read the value of the gEmergencyMemory global variable. The function SetA5 also
does two things: it returns the current value in the A5 register, and it sets the A5 register
to whatever value you pass to the function. The MyGrowZone function calls SetA5
with the original value of the A5 register as the parameter. In this case, the value returned
by SetA5 is ignored. There is no way to test whether, at the time your callback routine
is called, your application is executing a Toolbox routine that could change the A5
register. Therefore, to be safe, you should save and restore the A5 register in any callback
routine that accesses any part of your A5 world. Such routines include
grow-zone functions
Sound Manager completion routines n File Manager I/O completion routines
control-action procedures
TextEdit word-break and click-loop routines
trap patches
custom menu definition, window definition, and control definition procedures
Accessing the A5 World in Interrupt Tasks 4
Sometimes, an application-defined routine executes at a time when you can’t reliably call
SetCurrentA5. For example, if your application is not the current application and you
call SetCurrentA5 as illustrated in Listing 4-1, the function will not return your
application’s value of CurrentA5. The SetCurrentA5 function always returns the
value of the low-memory global variable CurrentA5, which always belongs to the
current application. You’ll end up reading some other application’s A5 world.
In general, you cannot reliably call SetCurrentA5 in any code that is executed in
response to an interrupt, including the following:
1. Time Manager tasks
2. VBL tasks
3. tasks installed using the Deferred Task Manager
4. Notification Manager response procedures
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Instead of calling SetCurrentA5 at interrupt time, you can call it at noninterrupt time
when yours is the current application. Then store the returned value where you can read it
at interrupt time. For example, the Notification Manager allows you to store information
in the notification record passed to NMInstall. When you set up a notification record,
you can use the nmRefCon field to hold the value in the A5 register.
Listing below illustrates how to save the current value in the A5 register and pass that
value to a response procedure.
Listing : Passing A5 to a notification response procedure
VAR
gMyNotification: NMRec; {a notification record}
BEGIN
WITH gMyNotification DO
BEGIN
qType := ORD(nmType); {set queue type}
nmMark := 1; {put mark in Application menu}
nmIcon := NIL; {no alternating icon}
nmSound := Handle(-1); {play system alert sound}
nmStr := NIL; {no alert box}
nmResp := @SampleResponse; {set response procedure}
nmRefCon := SetCurrentA5; {pass A5 to notification task}
END;
END;
4
The key step is to save the value of CurrentA5 where the response procedure can find
it— in this case, in the nmRefCon field. You must call SetCurrentA5 at
noninterrupt time; otherwise, you cannot be certain that it will return the correct value.
When the notification response procedure is executed, its first task should be to call the
SetA5 function, which sets register A5 to the value stored in the nmRefCon field. At
the end of the routine, the notification response procedure should call the SetA5
function again to restore the previous value of register A5. Listing below shows a simple
response procedure that sets up the A5 register, modifies a global variable, and then
restores the A5 register.
Listing : Setting up and restoring the A5 register at interrupt time
PROCEDURE SampleResponse (nmReqPtr: NMRecPtr);
VAR
oldA5: LongInt; {A5 when procedure is called}
BEGIN
oldA5 := SetA5(nmReqPtr^.nmRefCon);
{set A5 to the application’s A5}
gNotifReceived := TRUE; {set an application global }
{ to show alert was received}
oldA5 := SetA5(oldA5); {restore A5 to original value}
END;
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Conclusion
After going through this article, you might have got a broad idea of what memory
management in Mac alike and how it differs from the way other operating systems
handles memory.
For more insight into Mac memory management concepts and associated Toolbox
routines refer to:
http://developer.apple.com/techpubs/macos8/OSSvcs/MemoryManager/memorymanager.
html
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